HTGR power plant hot reheat steam pressure control system

ABSTRACT

A control system for a high temperature gas cooled reactor (HTGR) power plant is disclosed wherein such plant includes a plurality of steam generators, each deriving heat from a respective circulating flow of reactor coolant gas to supply superheated steam to a common main steam header and reheated steam to a common hot reheat header. Dual turbine-generators are connected to the common headers, a high pressure element of each turbine receiving steam from the main steam header, and an intermediate-low pressure element of each turbine receiving steam from the hot reheat header. Associated with each high pressure element is a bypass line connected between the main steam header and a cold reheat header, which is commonly connected to the high pressure element exhausts. Associated with each intermediate-low pressure element is a first bypass line connected between the hot reheat header and a respective condenser, and a second bypass line connected between the hot reheat header and an alternate steam receiving means such as a secondary condenser or atmosphere. The reactor coolant gas is circulated through each steam generator by an associated helium circulator which is rotated by an auxiliary steam turbine connected between the cold reheat header and the reheater section of the respective steam generator. A control system governs the flow of steam through the first and second bypass lines to provide for a desired minimum steam flow through the steam generator reheater sections at times when the total steam flow through the turbines is less than such minimum, and to regulate the hot reheat header steam pressure to improve control of the auxiliary steam turbines and thereby improve control of the reactor coolant gas flow, particularly following a turbine trip. When the desired steam flow through an intermediate-low pressure turbine is less than the predetermined minimum, the control system governs the flow of steam through the first bypass line so that the combined turbine and bypass steam flow is equal to the predetermined minimum. The steam flow through the second bypass line is controlled to regulate the flow of steam through the first bypass line at a predetermined variable limit value. At times when there is a difference between detected and desired values of hot reheat header steam pressure, the control system reduces such difference by varying the steam flow through one of the bypass lines in proportion to such difference. The steam flow through the second bypass line is varied only when the flow of steam through the first bypass line is at the predetermined variable limit.

United States Patent n91 Braytenbah et al.

[ HTGR POWER PLANT HOT REHEAT STEAM PRESSURE CONTROL SYSTEM [75] Inventors: Andrew S. Braytenbah, Pennsauken,

N.l.; Karl O. .laegtnes, Chester, Pa.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

22 Filed: Apr. 22, I974 21 Appl. No.: 463,027

[52] US. Cl. 60/644; 60/661; 60/663; 60/677 [51] Int. Cl. Folk 7/24 [58] Field of Search 60/660, 661, 663, 677, 60/679, 662, 644

[56] References Cited UNITED STATES PATENTS 3,055,l8l 9/1962 Argersinger 60/663 X 3,097,490 7/l963 Callan et al 60/663 X FOREIGN PATENTS OR APPLICATIONS l,40l,447 4/l962 Germany 60/663 Primary Examiner-Martin P. Schwadron Assistant Examiner-Allen M. Ostrager Attorney, Agent, or Ft'rm-E. F. Possessky [57] ABSTRACT A control system for a high temperature gas cooled reactor (HTGR) power plant is disclosed wherein such plant includes a plurality of steam generators, each deriving heat from a respective circulating flow of reactor coolant gas to supply superheated steam to a common main steam header and reheated steam to a common hot reheat header. Dual turbine-generators are connected to the common headers, a high pressure element of each turbine receiving steam from the main steam header. and an intermediate-low pressure element of each turbine receiving steam from the hot July 15, 1975 reheat header. Associated with each high pressure element is a bypass line connected between the main steam header and a cold reheat header, which is commonly connected to the high pressure element exhausts. Associated with each intermediate-low pressure element is a first bypass line connected between the hot reheat header and a respective condenser, and

a second bypass line connected between the hot re- 7 heat header and an alternate steam receiving means such as a secondary condenser or atmosphere. The reactor coolant gas is circulated through each steam generator by an associated helium circulator which is rotated by an auxiliary steam turbine connected between the cold reheat header and the reheater section of the respective steam generator. A control system governs the flow of steam through the first and second bypass lines to provide for a desired minimum steam flow through the steam generator reheater sections at times when the total steam flow through the turbines is less than such minimum, and to regulate the hot reheat header steam pressure to improve control of the auxiliary steam turbines and thereby improve control of the reactor coolant gas flow, particularly following a turbine trip. When the desired steam flow through an intermediate-low pressure turbine is less than the predetermined minimum, the control system governs the flow of steam through the first bypass line so that the combined turbine and bypass steam flow is equal to the predetermined minimum. The steam flow through the second bypass line is controlled to regulate the flow of steam through the first bypass line at a predetermined variable limit value. At times when there is a difference between detected and desired values of hot reheat header steam pressure, the control system reduces such difference by varying the steam flow through one of the bypass lines in proportion to such difference. The steam flow through the second bypass line is varied only when the flow of steam through the first bypass line is at the predetermined variable limit.

49 Claims, 4 Drawing Figures com REHEAY HEADER MAIN srzm ma L I 05 og larger mime t; CZVW+ BE I06 I07 loa l o B'YPlSS VALVE ONT'ROL SYSTEM HTGR POWER PLANT HOT REHEAT STEAM PRESSURE CONTROL SYSTEM CROSS REFERENCES TO RELATED APPLICATIONS Arrangement for Controlling the Loading of a Turbine System, Aanstad, O.J., Ser. No. 369,332, filed June 12, I973.

Acceleration Control Arrangement for Turbine System, Aanstad, 0.1.. Ser. No. 367,991, filed June 7, 1973.

BACKGROUND OF THE lNVENTlON in an HTGR power plant a cooling gas (helium) is circulated through the reactor whenever the reactor operates. In an indirect cycle HTGR power plant, hot reactor cooling gas flows from the reactor to the primary sides of a plurality of steam generators which derive heat from the gas, and supply superheated and reheated steam to a turbine-generator. For desirable operation and protection of the steam generators it is necessary to maintain a minimum steam flow through the superheater and reheater sections of each steam generator. Typically, the total minimum steam flow through the steam generators is sufficient to generate 25% of maximum plant power. Therefore, bypass lines are connected across the various turbine elements to permit the total minimum steam flow through the steam generators at times when the steam flow through the turbine elements is less than such minimum.

A helium circulator is associated with each steam generator to circulate a cooling gas through the reactor and the respective steam generator. Such a circulator may be rotated by an auxiliary steam turbine. When auxiliary steam turbines are so utilized, an auxiliary turbine provided for each helium circulator uses a portion of the steam flowing to the inlet of the reheater section of the associated steam generator. The outlets of the reheater sections are commonly connected to a hot reheat header. Reheated steam may flow from the hot reheat header in three paths. A first path comprises an intercept valve and a lower pressure turbine element, a second path comprises a condenser bypass line and bypass valve means, and a third path comprises an alternate bypass line and valve means therein connected. Regulation of the hot reheat header steam pressure improves control of the shaft speeds of the auxiliary steam turbines and thus permits improved control of the flow rates of reactor coolant gas.

One proposed control system varies the position of the bypass valve means in a turbine bypass line to reduce the difference between detected and desired values of hot reheat header steam pressure. In this control system the position of the bypass valve means is varied in accordance with a signal which comprises a first component proportional to the pressure difference and a second component proportional to the time integral of the pressure difference. Such a system operates satisfactorily in an HTGR power plant which includes a single turbine-generator. However, such a control system has certain limitations when two such control systems operate in concert, as in an HTGR power plant which includes two bypass systems and two turbinegenerators, for example. When two such systems are so utilized two integrators simultaneously integrate a pressure difference signal, and the integrator output signals, which ideally are equal, frequently diverge in practice causing undesirable imbalance between the steam flows through the bypass lines.

In a prior art system for governing turbine bypass flow from a hot reheat header in an HTGR power plant an intercept and a hypass flow valve are interlocked, whereby one valve opens as the other valve closes. ldeally, the two valves are designed to present a constant total resistance to steam flow at any interlocked position so that the steam pressure in the hot reheat header remains substantially constant under conditions of constant steam flow from the reheaters. In practice, the steam flow from the reheaters may vary, or the total resistance of the intercept and bypass valves may vary, causing variation of the hot reheat header steam pressure. ln event of a turbine trip, a pressure relief valve may open to relieve excessive hot reheat header steam pressure and close thereafter at a predetermined pressure level; however, such on-ofi control typically permits fluctuation of the post-trip hot reheat header steam pressure, with possible deterioration of the accuracy of control of the reactor coolant gas flow rates.

It is desirable to provide a hot reheat header steam pressure control system which may be used in a dual turbine HTGR power plant without causing unwanted imbalances between the turbine bypass line steam flows, and which also may be used in a single turbine plant. It is desirable to provide such a hot reheat header steam pressure control system which simultaneously positions a condenser bypass valve means and an alternate bypass valve means in both a single and a dual turbine HTGR power plant. it is further desirable to operate the alternate bypass valve means to regulate the condenser bypass line steam flow at a limit which varies according to plant operating conditions. It is advantageous to provide a hot reheat header steam pressure control system which governs turbine bypass steam flow in response to a difference between detected and desired values of such pressure, as such a system controls the steam pressure despite variation of the steam flow through the reheaters. It is advantageous to provide a hot reheat header steam pressure control system which does not utilize interlocked control valves.

SUMMARY The present invention controls the steam pressure in a hot reheat header that is connected to the outlet of a reheating section of a steam source which is adapted to derive heat from the coolant gas of a high temperature nuclear reactor and arranged to supply super heated and reheated steam to a turbine-generator hav ing a high pressure turbine to utilize superheated steam and an intermediate-low pressure turbine to utilize steam from the hot reheat header, the coolant gas being circulated through the reactor and the steam source by a circulating means driven by an auxiliary steam turbine means connected to pass a portion of the steam flow to the inlet of the reheating section. At times when the desired steam flow through the intermediate-low pressure turbine is less than a predetermined minimum, the invention governs the steam flow through a bypass line connected between the hot reheat header and a condensing means so that the total steam flow from the hot reheat header is equal to the predetermined minimum. At low power output the invention thereby maintains the predetermined minimum steam flow through the reheating section for desirable operation and protection of the steam source. The present invention controls the steam pressure in the hot reheat header by varying the steam flow through the bypass line in predetermined proportion to a difference between detected and desired values of hot reheat header steam pressure to reduce the pressure difference. Thereby affording improved control of the rate of circulation of the reactor coolant gas.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a dual turbine HTGR power plant which includes a hot reheat header steam pressure control system according to one embodiment of the invention;

FIG. 2 is a block diagram of a bypass valve control system according to one embodiment of the invention; and,

FIGS. 3A and 3B graphically illustrate certain signals generated by the bypass valve control system of FIG. 2.

DETAILED DESCRIPTION Referring to FIG. I each of three helium circulators circulates helium coolant gas through a high temperature gas cooled reactor I and an associated steam generator. The steam generators 101A, 1018 and 101C are associated with the helium circulators 102A, 1028 and 102C respectively. Hot coolant gas is discharged from the reactor 100 and transports reactor-generated heat to each of the three steam generators. A steam generator derives heat from the reactor coolant gas flowing through it, to generate superheated and reheated steam. Feedwater is supplied to each of the steam generators through the line 103, and passes through economizer, evaporator and superheater sections in each steam generator. superheated steam is discharged from the steam generators through the lines 104A, 1048 and 104C, which conduct the superheated steam to a main steam header 105. Each steam generator also incorporates a reheater section, and utilizes reactor-generated heat to reheat a flow of steam through the incorporated reheater section. A dashed line illustrates the incorporation of a reheater section RHA in the steam generator 101A. Reheaters RHB and Rl-lC similarly are incorporated in the steam generators 1018 and 101C. Cold reactor coolant gas is discharged from a steam generator and pumped back through the reactor 100 by the associated helium circulator. It is understood that a typical HTGR power plant may employ a number different than three steam generators and associated helium circulators, depending upon the thermal generating capacity of the reactor 100. Additional steam generators would be connected to receive feedwater through the line 103 and to discharge superheated steam to the main steam header 105.

From the main steam header 105 steam may flow through a throttle valve 106 and a governor valve 107 to the inlet of a high pressure turbine 108. Exhaust steam from the high pressure turbine 108 is discharged to a cold reheat header 109. The high pressure turbine 108 turns on a shaft 110 with an intermediate pressure turbine 1 l l, a low pressure turbine 112 and a generator 113, hereafter referred to as the A turbine-generator. Bypass lines 114 and 115 are connected between the main steam header 105 and the cold reheat header 109, and bypass valves 1 l6 and l 17 are connected to govern the steam flows through the lines 114 and 115 respectively. Steam also may flow from the main steam header 109 through a throttle valve 118, a governor valve 119, and a highpressure turbine 120, to the cold reheat header 109. The high pressure turbine turns on a shaft 121 with an intermediate pressure turbine 122, a low pressure turbine 123, and a generator 124, hereafter referred to as the B turbine-generator. For most desirable steam generator operation, the steam flow through the superheater sections must be maintained at a level which is at least equal to a desired minimum steam flow. When the combined steam flow through the turbines 108 and 120 is less than the desired minimum, the bypass valves 116 and 117 are positioned to maintain the desired minimum steam flow through the superheater sections. At times when the combined steam flow through the turbines 108 and 120 exceeds the desired minimum, the valves 116 and 117 are closed. A similar desired minimum steam flow must be maintained through the reheater sections. For purposes of this discussion, the desired minimum steam flow is sufficient to generate 25% of maximum power plant output. It is understood that the power output corresponding to the desired minimum steam flow may vary, depending upon the particular design of the steam generators. It is recognized that each of the throttle valves 106 and 118 and each of the governor valves 107 and 119 corresponds to a plurality of such valves in typical practice.

An auxiliary steam turbine ASTA uses steam from the cold reheat header 109 to rotate the helium circulator 102A. Similarly auxiliary steam turbines ASTB and ASTC use steam from the cold reheat header 109 to rotate the helium circulators 102B and 102C respectively. A dashed line connecting the auxiliary steam turbine ASTC and the helium circulator 102C illustrates the rotational coupling of those elements. A control valve associated with each auxiliary steam turbine governs the steam flow through the auxiliary turbine, and thereby governs the rate of flow of reactor coolant gas through the corresponding helium circulator. Exhaust steam from the auxiliary steam turbine ASTA passes to the inlet of the reheater RIIA, and exhaust steam from the auxiliary steam turbines ASTB and ASTC similarly is discharged to the inlets of the respective reheaters RI-IB and RHC. A bypass line and bypass flow control valve are connected between the cold reheat header 109 and the inlet of each of the reheater sections RHA, Rl-IB and RHC. At times when the total steam flow into the cold reheat header 109 exceeds the total steam flow through the auxiliary steam turbines, the bypass valves associated with the auxiliary steam turbines are positioned such that the bypass lines conduct the excess steam flow directly to the reheater section inlets. A hot reheat header is connected to receive reheated steam from the outlets of the reheater sections. When more than three steam generators are utilized, the reheater section, the helium circulator and the auxiliary steam turbine corresponding to each additional steam generator are connected as above described.

From the hot reheat header I25 steam may flow through a stop valve 126 and an intercept valve 127 to the inlet of the intermediate pressure turbine 111. Exhaust steam from the turbine 111 flows through a line 128 to the inlet of the low pressure turbine 112. A line 129 conducts exhaust steam from the turbine 112 to a condenser 130. A condenser bypass line 131 is connected to conduct steam from the hot reheat header 125 to the condenser 130, and a condenser bypass valve 132 is connected to govern the steam flow through the line 131. An alternate bypass line 133 is connected between the hot reheat header 125 and an alternate steam receiving means, the alternate steam receiving means being atmosphere in FIG. 1. An alternate bypass valve 134 is connected to govern the steam flow through the line 133. The valve 132 is positioned by a valve positioner 135, preferably an electrohydraulic positioner which hydraulically moves the valve 132 to a position related to an electrical signal transmitted to the positioner 135 on a line 136. The valve 134 is positioned by a valve positioner 137, preferably an electrohydraulic positioner which positions the valve 134 at a position related to an electrical input signal transmitted to the positioner 137 on a line 138.

For purposes of this discussion, the stop valve 126 is assumed to be open, unless otherwise stated. Thus the rate of steam flow through the turbines 111 and 112 is governed by the intercept valve 127. A device 139 generates an output signal on a line 140 which represents a desired steam flow (intercept valve flow demand) through the turbines 111 and 112, and an electrohydraulic valve positioner 141 responds to the signal on the line 140 to position the valve 127 to cause a steam flow through such turbines effectively equal to the desired flow. The device 139 may be a manually set variable signal generator with an output on the line 140, or the device 139 may be a digital computer programmed to calculate a desired value of steam flow, which desired value is converted by an associated digital to analog converter and transmitted to the line 140. A device 142 generates an output signal on a line 143 which represents a desired value of steam pressure in the hot reheat header 125. The device 142 may be a manually set variable signal generator, with an output on the line 143 or it may be a digital computer programmed to calculate such a desired pressure value, the calculated value being converted by an associated digital to analog converter and transmitted to the line 143. A pressure transducer 144 detects the pressure of steam in the hot reheat header 125, and generates an output signal representative of the detected pressure on a line 145. A bypass valve control system 146 is responsive to the desired pressure signal on the line 143, the detected pressure signal on the line 145, and the desired steam flow signal on the line 140 to generate the valve positioner input signals on the lines 136 and 138 as hereinafter described.

Steam may flow through a stop valve 147 and an intercept valve 148 to the inlet of the intermediate pressure turbine 122. Exhaust steam from the turbine 122 flows through a line 149 to the inlet of the low pressure turbine 123. After flowing through the low pressure turbine 123, steam is conducted by a line 150 to a condenser 151. Condensed feedwater from the condensers 130 and 151 flows through a line 152 to a series of pumps and heaters (not shown). Heated and pressurized feedwater is supplied to the steam generators through the line 103.

A condenser bypass line 153 is connected between the hot reheat header 125 and the condenser 151, and a condenser bypass valve 154 is connected to govern the steam flow through the line 153. An alternate bypass line 155 is connected between the hot reheat header 125 and an alternate steam receiving means, the alternate means being atmosphere in FIG. 1. An alternate bypass valve 156 is connected to govern the steam flow through the line 155. An electrohydraulic valve positioner 157 positions the valve 154 at a position related to a signal on an input line 158. An electrohydraulic valve positioner 159 positions the valve 156 at a position related to a signal on an input line 160.

For the purposes of this discussion, it is assumed that the stop valve 147 is open, unless otherwise stated. Thus the rate of steam flow through the turbines 122 and 123 is governed by the intercept valve 148. A device 161 generates an output signal on a line 162 representing a desired steam flow (intercept valve flow demand) through the turbines 122 and 123. An electrohydraulic valve positioner 163 is responsive to the signal on the line 162 to position the intercept valve 148 to cause a steam flow through the turbines 122 and 123 effectively equal to the desired flow. The device 161 may be a manually set variable signal generator, or a programmed digital computer with an associated digital to analog converter. A device 163, which may be a manually set signal generator or a programmed digital computer with an associated digital to analog converter, generates a signal on a line 164 representative of a desired value of steam pressure in the hot reheat header 125. A bypass valve control system 165 is responsive to the detected pressure signal on the line 145, the desired pressure signal on the line 164, and the desired steam flow signal on the line 162 to generate the valve positioner input signals on the lines 158 and 160, as hereinafter described.

Although the stop valves 126 and 147 and the intercept valves 127 and 148 are illustrated as single valves in H6. 1, it is recognized that each valve corresponds to plurality of valves in typical practice.

Referring to FIG. 2, the bypass valve control system 146 is responsive to the intercept valve flow demand signal on the line to govern the steam flows through the condenser bypass line 131 and the alternate bypass line 133 at such rates that the combined steam flow through the turbines 111 and 112 and the bypass lines 131 and 133 is equal to one half the desired minimum steam flow through the reheater sections, at times when the steam flow through the turbines 111 and 112 is less than one half the desired minimum. Because the desired minimum steam flow is suffcient to generate 25% maximum plant power output, it follows that one-half the desired minimum steam flow is sufficient to generate 25% maximum power output of one turbine-generator, as the maximum power capabilities of the A" and 8" turbine-generators are equal. Thus, the steam flow through the turbines 111 and 112 is less than one half the desired minimum at times when the A turbine-generator is shut down, when the A trubine-generator is being accelerated prior to synchronization, after synchronization, When the power output of the A turbine-generator is less than 25% of its maximum power output, and following a trip of the A turbine-generator at a power output in excess of 25% of its maximum power output. The bypass valve control system 146 also responds to a difference between the desired and detected hot reheat header steam pressure signals on the respective lines 143 and to vary the steam flow through one of the bypass lines 131 and 133 to reduce such difference. Usually the bypass valve control system 146 holds the alternate bypass valve 134 closed and varies the steam flow through the condenser bypass line 133 to reduce a difference between the de sired and detected pressure signals. However, the bypass valve control system 146 opens the alternate bypass valve 134 to prevent the steam flow through the condenser bypass line 131 from exceeding a corresponding flow limit. When the alternate bypass valve 134 is open, the control system 146 positions the condenser bypass valve 132 to maintain the steam flow through the condenser bypass line 131 at the limit value, and varies the steam flow through the alternate bypass line 133 to reduce a difference between the desired and detected pressure signals.

In more detail with reference to FIG. 2 the intercept valve steam flow demand signal on the line 140 is transmitted through a multiplier 209 to a first input of a summing device 206. A bias signal generator 207 generates a constant bias signal which is connected to a second input of the summing device 206. A comparator 201 generates an output signal on a line 203 which is representative of the difference between the detected pressure signal on the line 145 and the desired pressure signal on the line 143. The signal on the line 203 is transmitted to a proportional controller 204, which generates an output signal connected on a line 205 to a third input of the summing device 206. The summing device 206 subtracts the output signal of the multiplier 209 from the constant bias signal, and adds to the difference of those signals the third input signal on the line 205, to generate an output signal on a line 210. The signal on the line 210 represents total bypass steam flow demand. to be satisfied by steam flow through the condenser bypass line 131 if that flow is less than a corresponding flow limit, or by the combined steam flow through the condenser bypass line 131 and the alternate bypass line 133, otherwise.

The line 210 is connected to a first input of a low select 211. A signal representing a limit value of steam flow through the condenser bypass line 131 is generated by a function generator 213 and is transmitted by a line 212 to a second input of the low select 211. If the total bypass steam flow demand signal is less than the condenser bypass flow limit signal, the low select 211 transmits the total bypass steam flow demand signal to the valve positioner 135, which positions the condenser bypass valve 132 to cause a flow of steam through the condenser bypass line 131 effectively equal to the total bypass steam flow demand, when the steam pressure in the header 125 is at a low load pressure value." If the total bypass steam flow demand signal exceeds the condenser bypass flow limit signal, the low select 211 transmits the condenser bypass flow limit signal to the valve positioner 135. A comparator 216 generates an output signal representing the excess of the total bypass steam flow demand over the condenser bypass flow limit, and the valve positioner 137 positions the alternate bypass valve 134 to cause a steam flow through the line 133 effectively equal to the flow represented by the output signal of the device 216. The valve positioner 135 positions the condenser bypass valve 132 to cause a flow of steam through the line 131 effectively equal to the condenser bypass flow limit. Then the combined steam flow through the lines 131 and 133 is effectively equal to the total bypass steam flow demand, when the steam pressure in the header 125 is at a low load pressure value." The output signal of the comparator 216 is zero whenever the total bypass steam flow demand is less than the condenser bypass flow limit, and the alternate bypass valve positioner 137 then holds the alternate bypass valve 134 closed.

Each of the bypass control valves 132 and 134 is characterized by a linear relationship between valve position and steam flow through the valve at constant valve inlet pressure. The valve positioner associated with each bypass control valve moves the respective valve to a position which is linearly related to the input signal to which the positioner responds. Bypass control valves having non-linear characteristics may be used; when such valves are used each valve positioner is modified to move the associated bypass control valve to a position which is non-linearly related to the respective valve positioner input signal, to compensate the non-linearity of the bypass control valve. A plurality of valves may be utilized to perform the function of the condenser bypass valve 132 or of the alternate bypass valve 134. In that instance a valve positioner is provided for each such valve, and the positioners operate in concert to cause a steam flow through the respective bypass line which is effectively equal to that when a single valve and associated valve positioner are used.

Provided that the detected pressure of steam in the hot reheat header does not deviate from the desired value, the input and output signals of the proportional controller 204 are zero, and the total bypass steam flow demand signal is a function solely of the intercept steam flow demand. 1f the detected pressure of steam in the header 125 differs from the desired pressure, a pressure difference signal is generated by the comparator 201 and is transmitted through the proportional controller 204 to the summing device 206, which modifies the total bypass steam flow demand signal according to the output signal of the controller 204. As the bypass valves 132 and 134 are positioned to satisfy the modified total bypass steam flow demand signal, the pressure difference is reduced.

The following equation relates to the function generator 213:

wherein BTU maximum allowable rate of heat delivery to the condenser by the condenser bypass line 131, K proportionality constant, F maximum steam flow through the condenser bypass line 131 corresponding to BTU HRHP hot reheat header 125 steam pressure. Hence F BTU /(K X l-lRl-IP). in the latter relationship the maximum steam flow in the condenser bypass line 131 varies inversely with the pressure of steam in the header 125. Therefore the function generator 213 is responsive to the output signal of the pressure transducer 144, which represents the detected pressure of steam in the header 125, to generate the signal on the line 212, which represents F according to the above relationship.

Referring to FIG. 3A the intercept valve steam flow demand signal on the line is graphically represented (line 300) in relation to the power output of the A turbine-generator. On the vertical axis the intercept valve steam flow demand is shown on a scale normalized between 0 and 1.0. On the horizontal axis the power output of the A turbine-generator is shown in percent of the maximum power output of that turbinegenerator. The intercept valve steam flow demand increases from 0 to 1.0 as the power output increases from O to 25%. An intercept valve flow demand of 0 causes the valve positioner 141 (see FIG. 1) to close the intercept valve 127. An intercept valve flow demand of 1.0 causes the valve positioner 141 to open fully the intercept valve 127. Over the power output range 25% to 100% the intercept valve flow demand is constant at 1.0, and the valve positioner 141 holds the intercept valve 127 fully open over such power output range. Between and 25% power output, the desired steam pressure in the hot reheat header 125 is regulated at a constant value (the low load pressure value") such that fully opening the intercept valve 127 causes a steam flow through the turbines 111 and 112 effectively equal to one-half the desired minimum steam flow through the reheater sections (see FIG. 1). In event that the device 139 is a manually set variable signal generator, an operator sets the device 139 to generate a signal on the line 140 of a value in accord with the heavy line 300 of FIG. 3A and the desired power output. As the power output of the A turbinegenerator increases from O to 25%, the corresponding steam flow through the turbines 111 and 112 increases from zero to one-half the desired minimum steam flow.

Again with reference to FIG. 3A, a dashed line 301 graphically represents the output signal of the multiplier 209 (see FIG. 2) in relation to the power output of the A turbine-generator. The multiplier 209 multiplies the intercept valve steam flow demand signal by a constant factor of 0.5, therefore the output signal of the multiplier 209 increases in value from 0 to 0.5 as the power output increases from 0 to 25%. Above 25% power output, the output signal of the multiplier 209 is constant at 0.5.

Referring now to FIG. 3B the total bypass steam flow demand signal on the line 210 (see FIG. 2) is graphically represented (line 302) in relation to the power output of the A turbine-generator. On the vertical axis the total bypass steam flow demand signal is shown on a scale between 0 and 0.5. On the horizontal axis the power of the A turbine-generator is shown in percent. The bias signal generated by the signal generator 207 (see FIG. 2) has a constant value of 0.5 in relation to the output signal of the multiplier 209, which is represented by the dashed line of FIG. 3A. Assuming that the difference between the detected and desired value of hot reheat header steam pressure is zero, the comparator 201 (see FIG. 2) generates a zero output signal on the line 203, and the signal on the line 205 accordingly is zero. Then the total bypass steam flow demand signal is generated by the summing device 206 (see FIG. 2) according to the difference between the constant bias signal of value 0.5 and the output signal of the multiplier 209. As shown in FIG. 3B the total bypass steam flow demand signal decreases from 0.5 to 0 as the power output of the A turbine-generator increases form 0 to 25%. Above 25%. Above 25% power output, the total bypass steam flow demand is constant at 0. A total bypass steam flow demand of 0.5 causes the condenser bypass valve positioner 135 to position the condenser bypass valve 132 such that the steam flow through the condenser bypass line 131 is effectively equal to one-half the desired minimum steam flow, when the pressure of steam in the hot reheat header 125 is at the low load pressure value, and the condenser bypass flow limit is greater than one half the desired minimum steam flow. Otherwise the valve positioners 135 and 137 position the bypass valves 132 and 134 so that the combined steam flow through the condenser bypass line 131 and the alternate bypass line 133 is effectively equal to one half the desired minimum, when the hot reheat steam pressure is at the low load value. A total bypass steam flow demand of 0 causes the valve positioners and 137 to hold the bypass valves 132 and 132 closed. Thus the combined steam flow through the bypass lines 131' and 133 decreases from one half the desired minimum steam flow to zero, as the power output of the A turbine-generator increases from 0 to 25%, on the assumption that no pressure difference signal is generated by the comparator 201.

The heavy line of FIG. 3A shows a linear relationship between power output and intercept valve flow demand between 0 and 25% maximum power output for purposes of clarity and simplicity of exposition, and should not be construed as a limitation. The bypass valve control system 146 is equally effective in response to a non-linear relationship between power output and intercept valve flow demand over such power output range as long as the intercept valve is fully opened at 25% maximum power output.

It is understood that values other than 0.5 may be used for the gain of the multiplier 209 and the value of the bias signal. For example, the bias signal value and the multiplier gain may each be 1.0, in which case the line would be connected directly to the summing device 206 and the valve positioners 135 and 137 would be arranged to position the respective valves 132 and 134 to cause a total steam flow through the lines 131 and 133 which is effectively equal to one half the desired minimum steam flow when the total bypass flow demand is 1.0 and the hot reheat header steam pressure is at the low load pressure value. The value 0.5 is suitable when the condenser 135 is capable of con densing the total desired minimum steam flow at the low load pressure value of hot reheat steam pressure.

Over the power output range 0 to 25% the output signal of the device 142 represents a constant desired pressure equal to the low load pressure value. From the above discussion, it is evident that over such power output range the bypass valve control system 146 governs the steam flow through the condenser bypass line 131 and the alternate bypass line 133 so that the combined steam flow through such bypass lines and the turbines 111 and 112 is effectively equal to one half the desired minimum steam flow, assuming no difference between the detected and desired values of hot reheat header steam pressure. Between power output levels of 0 and 25% an increase of steam flow through the turbines 111 and 112 is accompanied by a corresponding decrease of steam flow through the bypass lines 131 and 133, and in effect the bypass control system 146 transfers bypass steam flow to the turbines 111 and 112 as the power output of the A turbine-generator increases. while regulating the steam pressure in the hot reheat header 125.

If such a difference occurs, the bypass valve control system 146 varies the steam flow through one of the bypass lines 131 and 133 to reduce the difference. In practice such a pressure difference cannot be reduced to zero, as the controller 204 is a proportional controller and permits a residual pressure difference. However, the value of the bias signal generated by the signal generator 207 is such that the magnitude of the residual pressure difi'erence is effectively minimized.

Between 0 and 25% maximum total plant power output the reactor 100 and the helium circulators 102A-102C are operated by controls (not shown) so that the reheater sections of the steam generators are capable of supplying the desired minimum flow of reheated steam when the hot reheat header steam pressure is at the low load pressure value. Then the bypass valve control system 146 regulates the steam pressure in the header 125 according to the low load pressure value and simultaneously governs the steam flow through the bypass lines 131 and 133 so that the combination of steam flows through such lines with the steam flow through the turbines 111 and 112 effectively equals one-half the desired minimum steam flow. If the reactor 100 and the helium circulators 102A-102C are not operated to supply the desired minimum flow of reheated steam at the low pressure value, the bypass valve control system 146 varies the steam flow through the bypass lines 131 and 133 to regulate the steam pressure in the hot reheat header 125 according to the desired low load value, but the total steam flow through the bypass lines 131 and 133 and the turbines 111 and 112 differs from one half the desired minimum steam flow by an amount which depends upon the operation of the reactor and helium circulators.

With reference to FIG. 2 identification numbers in parentheses refer to the bypass valve control system 165 associated with the B turbine-generator. The elements and connection of the bypass valve control system 165 are shown within the dashed lines in FIG. 2. The above description of the connection and operation of the bypass valve control system 146 also relates to the control system 165 provided that the numbers in parentheses are substituted for the corresponding numbers in the text, and that the expression turbines 122 and 123 is substituted for the expression turbines 111 and 112.

In one mode of operation the A and B turbinegenerators are loaded simultaneously (after synchroni zation) between and 25% maximum plant power output. In this mode each of the devices 142 and 163 generates an output signal representative of the low load pressure value. It is understood that in the mode of operation presently being described a single device may be utilized to generate the desired hot reheat header steam pressure signal on the lines 143 and 164. As hereinafter described, other modes of operation require the two devices 142 and 163 to generate independent signals on the lines 143 and 164. The intercept valve flow demand signals on the lines 140 and 162 simultaneously increase between 0 and I at a rate such that the turbines are protected from undesirable thermal stress. In response, the intercept valve positioners 141 and 163 increasingly open the respective intercept valves 127 and 148 to increase the steam flows through the intermediate pressure turbines 111 and 123 in accordance with the intercept valve flow demand signals. At any intercept flow demand value between 0 and l the multiplier 209 (see FIG. 2) in each of the bypass valve controllers transmits the respective intercept valve flow demand signal with a gain of one half to the summing device 206, which subtracts the multiplier output signal from the constant bias signal (assuming that the hot reheat header steam pressure is at the low load pressure value) to generate thetotal bypass flow demand value which corresponds to the respective intercept valve flow demand value. The sum of the intercept valve flow demand with the total bypass flow demand is a demand steam flow equal to one half the desired minimum steam flow through the reheaters. In each bypass valve control system, the low select passes the total bypass flow demand to the condenser bypass valve positioner, which positions the condenser bypass valve to cause a flow of steam through the condenser bypass line equal to the total bypass flow demand when such demand is less than the condenser bypass flow limit and the hot reheat header steam pressure is at the low load pressure value. If the total bypass flow demand exceeds the condenser bypass flow limit, the low select transmits the condenser bypass flow limit to the condenser bypass valve positioner while the comparator 216 transmits the difference between the total bypass flow demand and the condenser bypass flow limit to the alternate bypass valve positioner. The condenser and alternate bypass valve positioners position the bypass valves to cause steam flows through the bypass lines in accordance with the respective valve positioner input signals, and the combined steam flow through the bypass lines is effectively equal to the total bypass flow demand. When the hot reheat header steam pressure is at the low load pressure value. When steam flow through the alternate bypass line is required to satisfy the total bypass flow demand, the condenser bypass steam flow is regulated at the corresponding flow limit, thereby minimizing the flow of steam through the alternate bypass line to atmosphere. At any power output between 0 and 25% the bypass valve control systems operate the bypass valves in concert so that the flow of steam through each bypass system (comprising one condenser bypass line and one alternate bypass line) is effectively equal to one half the difference between desired minimum steam flow and the total steam flow through the turbines 111 and 122. Thus the bypass valve control systems operate the associated bypass valves to maintain the desired minimum steam flow through the reheaters between 0 and 25% maximum plant power, when the hot reheat header pressure is at the low load pressure value.

If the steam generators cannot supply the desired minimum flow of reheated steam at the low load pressure value the comparator 201 in each bypass valve control system generates a pressure difference signal which is transmitted through the proportional controller 204 to the summing device 206, which modifies or trims the total bypass flow demand signal according to the controller output signal. When the bypass valves are positioned to cause a total bypass flow in accordance with the modified total bypass flow demand, the pressure difference is reduced. If the detected pressure exceeds the low load pressure value for example, the trim signal on the line 205 is positive, thereby increasing the total bypass flow demand to cause a reduction of the pressure difference when the bypass valves are positioned in response to such increased demand. The pressure detector 144, the comparator 201, and the proportional controller 204 thus comprise a pressure feedback path which trims the total bypass flow demand (line 302, FIG. 38) to reduce a difference between the detected and desired values of hot reheat header steam pressure. In event that the trimmed total bypass flow demand signal exceeds the corresponding condenser bypass flow limit signal, the low select operates to govern the condenser bypass line flow at its corresponding flow limit, while the trimmed total flow demand is satisfied by alternate bypass steam flow, as heretofore described. Thus the bypass valve control systems operate their associated bypass valves in concert to vary the total bypass flow from the hot reheat header to reduce a difference between detected and desired hot reheat header steam pressure values.

When an integral mode is incorporated in each of the controllers 204, the output signals of the integrators (which ideally are equal) diverge in practice as the integrators individually integrate various disturbances which may affect one but not both of the integrators. Such divergence causes unwanted imbalance between the total bypass flow demand signals, which otherwise would be equal in the above-described simultaneous loading of dual turbine-generators. Although the hot reheat header steam pressure is effectively controlled in the presence of such imbalances, an imbalance may cause one of the total bypass flow demand signals to exceed its corresponding condenser bypass flow limit, resulting in unnecessary and unwanted discharge of steam to atmosphere. Because the controller 204 incorporates a proportional mode rather than a combination of proportional and integral modes, imbalances of the total bypass steam flow demand signals which result from integration of various disturbances are desirably eliminated. While the proportional mode controllers 204 typically permit a residual difference between the detected and desired values of hot reheat header steam pressure, such residual differences are minimized by the bias signals.

As the power output is increased in the above example of simultaneous loading of dual turbine-generators, the intercept valves are increasingly opened until they are fully open at 25% maximum power output. Correspondingly, the bypass valves are increasingly closed until they are effectively fully closed at 25% maximum power output. Above 25% maximum plant power output, the pressure of the hot reheat header 125 is permitted to increase with increasing load, and the bypass valve control systems 146 and 165 are operated in the tracking mode wherein the devices 142 and 163 generate output signals which are identical to the output signal of the pressure detector 144. Referring to FIG. 2, tracking mode operation at power output in excess of 25% maximum plant power assures that the bypass valves 132, 124, 154, and 156 remain closed, because the total bypass steam flow demand (see FIG. 38) at such power levels, in absence of a pressure difference signal on the line 203 is zero.

When both turbine-generators operate and one turbine-generator is tripped the stop valve associated with the tripped turbine is closed (by controls not shown). Then only one-half of the steam flow through the reheaters is required by the operating turbine, and the remainder of the reheated steam must be bypassed in order to stabilize the hot reheat header post-trip steam pressure. In the event that each turbinegenerator operated at a power output greater than 25% of its maximum output prior to the trip, the bypass valve control system associated with the operating turbine remains in the tracking mode in order that none of the excess reheated steam is bypassed to the condenser associated with the operating turbine. After the trip the desired pressure signal associated with the tripped turbine bypass valve control system continues to represent the hot reheat header steam pressure immediately before the trip. The bypass valve control system associated with the tripped turbine responds to an intercept valve flow demand of zero (that value corresponding to zero output power as shown in FIG. 3A line 300) and generates a total bypass steam flow demand of value 0.5 (assuming zero pressure difference). The low select transmits flow demand signals to one or both of the condenser and alternate bypass valve positioners as heretofore described, and the valve positioners position the bypass valves according to the valve positioner input signals to cause a flow of bypass steam effectively equal to one half the desired minimum steam flow were the hot reheat header steam pressure at the low load pressure value. If the resulting bypass steamflow is not effectively equal to the reheated steam flow which does not pass through the operating turbine, a difference develops between the detected and desired values of hot reheat header steam pressure, and a difference signal is generated by the comparator 201 of the bypass valve control system associated with the tripped turbine. Then the total bypass steam flow demand is trimmed by the summing device 206 in accordance with the output signal of the controller 204 to cause a reduction of the pressure difference when the bypass valves are positioned in response to the trimmed total bypass steam flow demand. The low select operates to open the alternate bypass valve only when the total bypass flow demand exceeds the condenser bypass flow limit and governs the condenser bypass flow at the flow limit at such times, to minimize the steam which is discharged to atmosphere. The post-trip steam pressure in the hot reheat header thus is stabilized at a value close to the pressure value which prevailed prior to the trip. Posttrip pressure stabilization is advantageous because the operating turbine-generator continues to generate power at its desired power output level without sudden change of the positions of the control valves associated with the operating turbine, and without transient fluctuations of the power generated by the operating turbine-generator which otherwise may result from large post-trip transient excursions of the steam pressure in the hot reheat header. Such pressure stabilization also reduces post-trip transient variation of the shaft speeds of the auxiliary steam turbines (see FIG. 1) and thereby reduces post-trip variation of the reactor coolant gas flow rates.

In event that both turbine-generators are simultaneously tripped at a power output in excess of 25% maximum power output, each of the devices 142 and 163 generates an output signal after the trip which is representative of the hot reheat header steam pressure immediately before the trip. After the trip the intercept valve flow demand signals on the lines and 162 are zero, while the detector 144 generates an output signal representative of the post-trip steam pressure in the hot reheat header. The bypass valve control systems 146 and operate the respective bypass valves in concert to bypass the steam flow from the reheaters, and thereby regulate the hot reheat header pressure at the pre-trip pressure value. Single and dual turbine trips may occur at power output levels, below 25% maximum power output. The bypass valve control systems operate the bypass valves as above described to regulate the post-trip hot reheat header steam pressure, the difference being that the pressure is regulated at the low load pressure value following a trip at such lower power levels.

The bypass valve control system shown in FIG. 2 also may be applied to operate the bypass valves in condenser and alternate bypass lines associated with a single full size turbine-generator. In such an application, the intercept valve passes the desired minimum steam flow through the intermediate-low pressure turbines when it is fully open and the hot reheat header steam pressure is at the low load pressure value, and the condenser and alternate bypass valves are arranged so that either bypass line would conduct the desired minimum steam flow when the value of the total bypass steam flow demand signal is 0.5 and the hot reheat header steam pressure is at the low load pressure value.

The single full size turbine-generator is loaded (after synchronization) between and 25% maximum power output while the hot reheat header steam pressure is controlled at the low load pressure value. The desired hot reheat header steam pressure signal on the line 145 (see FIG. 2) represents the low load pressure value, and the intercept valve flow demand signal on the line 140 increases from O to l .0 at a rate such that the turbine is not subjected to undesirable thermal stress. The intercept valve is positioned in accordance with the intercept valve flow demand signal and the steam flow through the intermediate-low pressure turbine correspondingly increases with the intercept valve flow de mand signal, the steam flow through the intermediatelow pressure turbine being effectively equal to the desired minimum steam flow when the intercept valve flow demand is ID and the hot reheat header steam pressure is at the low load pressure value. The total bypass flow demand signal (see FIG. 3B) generated in response to the intercept valve flow demand (assuming the hot reheat header steam pressure is at the low load pressure value) is such that the sum of the intercept valve flow demand with the total bypass flow deamnd is equal to the desired minimum flow. The low select 211 (see FIG. 2) transmits the total bypass flow demand signal to the condenser bypass valve positioner when the total bypass flow demand is less than the condenser bypass flow limit, otherwise the low select 21] transmits the condenser bypass flow limit signal to the condenser bypass valve positioner, while the comparator 216 transmits the difference between total bypass flow demand and the condenser bypass flow limit to the alternate bypass valve positioner. The condenser bypass valve is positioned to cause a flow effectively equal to the total bypass flow demand when the total bypass flow demand is less than the condenser bypass flow limit and the hot reheat header steam pressure is at the low load pressure value. The alternate bypass valve is closed at such times. When the total bypass flow demand is greater than the condenser bypass flow limit, the flow through the condenser bypass line is regulated at the flow limit, while the alternate bypass valve is positioned such that the total steam flow through the condenser and alternate bypass lines equal to the total bypass flow demand when the hot reheat header steam pressure is at the low load pressure value. Thus the steam discharge to atmosphere is minimized at times when the alternate bypass valve must be opened. At any intercept valve flow demand between 0 and 1.0, the bypass valve control system 146 operates the condenser and alternate bypass vavles in response to the intercept valve flow demand to cause a total steam flow through the bypass lines such that the total flow from the hot reheat header is efficiently equal to the desired minimum flow when the hot reheat header steam pressure is at the low load pressure value.

If the steam generators cannot supply the desired minimum flow of reheated steam at the low load pressure value as the turbine-generator is loaded between O and 25% maximum power output, a pressure difference signal is generated by the comparator 201 (see FIG. 2) and the summing device 206 modifies the total bypass flow demand according to the trim signal generated by the proportional controller 204 on the line 205. When the condenser and alternate bypass valves are positioned in response to the modified total bypass flow demand, the difference between detected and desired values of hot reheat header steam pressure is reduced. When the alternate bypass valve is closed, the steam flow through the condenser bypass line is varied to reduce the pressure difference. When the modified total bypass flow demand exceeds the condenser bypass flow limit the steam flow through the alternate bypass line is varied to reduce the pressure difference, as the condenser bypass valve positioner input signal is constant at such times. Although the proportional controller 204 permits a residual difference between detected and desired values of hot reheat header steam pressure, the residual difference is effectively minimized by the bias signal (see FIG. 2).

At 25% maximum power output the condenser and alternate bypass valves are effectively fully closed. Above 25% maximum power output, the hot reheat header steam pressure increases with increasing load, and the bypass valve control system 146 (see FIG. 2) is operated in the tracking mode, whereinthe detected and desired pressure signals on the respective lines and 143 are equal, to ensure that the condenser and alternate bypass valves remain closed. After a turbine trip at a power output level in excess of 25% maximum power output, the desired pressure signal on the line 143 continues to represent the hot reheat header steam pressure immediately before the trip. The detected pressure signal on the line 145, however, represents the hot reheat header steam pressure as a result of the trip. The stop valve associated with the intermediate-low pressure turbine is closed (by means not shown) when the turbine is tripped, and the entire flow of reheated steam to the hot reheat header must be bypassed. The post-trip intercept valve flow demand signal is 0 (see FIG. 3A), corresponding to 0 power output, and the total bypass steam flow demand (see FIG. 3B) is 0.5, assuming no pressure difference signal on the line 203 (see FIG. 2). Hence the bypass valves are positioned to cause a total bypass steam flow equal to the desired minimum steam flow were the hot reheat header steam pressure at the low load pressure value. If the total bypass steam flow is not equal to the flow of reheated steam, a difference develops between the detected and desired values of hot reheat header steam pressure, and the comparator 201 generates a pressure difference signal on the line 203 which is transmitted through the proportional controller 204 to the summing device 206. The summing device 206 modifies the total bypass flow demand according to the output signal of the proportional controller 204 on the line 205, and the pressure difference is reduced when the condenser and alternate bypass valves are positioned in accordance with the modified total bypass flow demand. Such post-trip regulation of the hot reheat header steam pressure reduces post-trip transient variation of the shaft speeds of the auxiliary steam turbines (see FIG. 1) and thereby reduces post-trip variation of the corresponding reactor coolant gas flow rates. The bypass valve control system also operates the bypass valves to minimize variation of the hot reheat header steam pressure following a turbine trip at a power output less than 25% maximum power output, in that event, the post-trip hot reheat header steam pressure is regulated at the low load pressure value.

It should be understood that various modifications changes and variations may be made in the arrangement, operation and details of construction of the elements herein disclosed without departing from the spirit and scope of the present invention.

We claim:

1. A system for controlling steam pressure in a hot reheat header connected to receive reheated steam from a reheating section of a steam source that is adapted to derive heat from a flow of reactor coolant gas and to supply superheated and reheated steam to a turbine-generator including at least a high pressure and an intermediate-low pressure turbine, said gas being circulated through a high temperature nuclear reactor and said steam source by a circulating means rotated by an auxiliary steam turbine means connected to pass a portion of the steam which flows to the inlet of the reheating section, and said hot reheat header being connected so that steam may flow from the header to a condensing means through a first path comprising the intermediate-low pressure turbine and through a second path comprising a condenser bypass line, said control system comprising,

valve means connected to govern the steam flow through the condenser bypass line,

means to generate a first signal representative of a desired steam flow through the intermediate-low pressure turbine,

pressure detecting means to generate a second signal representative of a detected value of steam pressure in the hot reheat header,

means to generate a third signal representative of a desired value of steam pressure in the hot reheat header, and

control means responsive to the first signal to position the valve means to govern the steam flow through the condenser bypass line to maintain a steam flow through the reheating section that is equal to a predetermined minimum flow at times when the detected and desired pressure values are equal to a predetermined low load pressure value and the desired steam flow through the intermediate-low pressure turbine is less than the predetermined minimum flow, and responsive to the second and third signals when the second and third signals are different to vary the steam flow through the condenser bypass line in proportion to the difference of the second and third signals to reduce said difference.

2. A control system according to claim 1 wherein the control means include.

means to generate a fourth signal having a predetermined proportionality with the difference between the second and third signals,

means to generate a fifth signal having a predetermined proportionality with the first signal,

means to generate a sixth signal representative of the fourth signal diminished by the fifth signal, and means to position the valve means in accordance with the sixth signal.

3. A control system according to claim 2 wherein the means to generate the fourth signal comprise,

a comparator to generate an output signal representative of the difference between the second signal and the third signal, and

a proportional controller responsive to the output signal of the comparator to generate the fourth signal, said fourth signal having a predetermined proportionality with the comparator output signal.

4.'A control system according to claim 1 wherein the valve means is characterized by a linear relationship between steam flow through the valve means and position of the valve means at constant inlet pressure to the valve means, and wherein the associated positioning means positions the valve means at a position which is linearly related to the sixth signal.

5. A control system according to claim 2 wherein the control means further include means to generate a bias signal, and wherein the sixth signal represents the sum of the bias signal with the fourth signal, diminished by the fifth signal.

6. A control system according to claim 5 wherein the valve means is characterized by a linear relationship between steam flow through the valve means and position of the valve means at constant inlet pressure to the valve means, and wherein the associated positioning means positions the valve means at a position which is linearly related to the sixth signal.

7. A system for controlling steam pressure in a hot reheat header connected to receive steam from a reheating section of a steam source that is adapted to derive heat from a flow of reactor coolant gas and to supply superheated and reheated steam to a turbinegenerator including at least a high pressure and an intermediate-low pressure turbine, said gas being circulated through a high temperature nuclear reactor and said steam source by a circulating means rotated by an auxiliary steam turbine means connected to pass a portion of the steam which flows to the inlet of the reheating section, said hot reheat header being connected so that steam may flow from the header to a condensing means through a first path comprising the intermediate-low pressure turbine and through a second path comprising a condenser bypass line, an alternate bypass line being connected to pass steam from the hot reheat header to an alternate steam discharge means, said control system comprising,

first valve means connected to govern the steam flow through the condenser bypass line,

second valve means connected to govern the steam flow through the alternate bypass line,

means to generate a first signal representative of a desired steam flow through the intermediate-low pressure turbine,

pressure detecting means to generate a second signal representative of a detected value of steam pressure in the hot reheat header,

means to generate a third signal representative of a desired value of steam pressure in the hot reheat header, and

control means responsive to the first signal to position the first and second valve means to govern the steam flows through said bypass lines to maintain a steam flow through the reheating section that is equal to a predetermined minimum flow at times when the detected and desired pressure valves are equal to a predetermined low load pressure value and the desired steam flow through the intermediate-low pressure turbine is less than the predetermined minimum flow, and responsive to the second and third signals at times when the second and third signals are different to vary the total steam flow through said bypass lines to reduce said difference.

8. A control system according to claim 7 wherein the control means includes means to generate a signal representative of a limit value of steam flow through the condenser bypass line, and wherein the second valve means is positioned to regulate the condenser bypass line flow at the limit value, the steam flow through the alternate bypass line being varied to reduce said pressure difference at times when the condenser bypass line flow is so regulated.

9. A control system according to claim 8, wherein the flow limit signal is generated in accordance with the second signal, and the value of the limit signal varies inversely with the detected value of steam pressure in the hot reheat header.

10. A control system according to claim 9 wherein the means for generating the limit signal is a function generator, said function generator having the second signal as an input signal and generating the limit signal as an output signal.

11. A control system according to claim 7 wherein the control means include,

means to generate a fourth signal in accordance with the difference between the second and third signals,

means to generate a fifth signal having a predetermined proportionality with the first signal,

means to generate a bias signal,

means to generate a sixth signal representative of the sum of the bias signal with the fourth signal diminished by the fifth signal,

means to generate a signal representative of a limit valve of steam flow through the condenser bypass line, and

means to position the first and second valve means in response to the sixth signal and the limit signal.

12. A control system according to claim 11 wherein the fourth signal has a predetermined proportionality with the difference between the second and third signals.

13. A control system according to claim 11 wherein the means to position the first and second valve means include,

selection means responsive to the sixth signal and to the flow limit signal to select the lower of said signals and transmit the selected signal to a means for positioning the first valve means and to a comparator,

means to position the first valve means in accordance with the selected signal,

a comparator to generate an output signal representative of the difference between the sixth signal and the selected signal, and

means to position the second valve means in accordance with the output signal of the comparator.

14. A control system according to claim 13 wherein the second valve means is closed at times when the sixth signal is lower than the limit signal.

15. A control system according to claim 14 wherein the first valve liieans is closed when the sixth signal is zero.

16. A clltfbl system according to claim 13 wherein each of fllfi first and second valve means is characterized by a linear relationship between steam flow through the valve means and position of the valve means at constant difierential pressure across the valve means, and wherein the associated positioning means positions the valve means at a position which is linearly related to the signal to which the positioning means is responsive.

17. A system for controlling steam pressure in a hot reheat header connected to receive reheated steam from a reheating section of a steam source which is adapted to derive heat from a flow of reactor coolant gas and to supply superheated and reheated steam to first and second turbine-generators, each of said first and second turbine-generators including at least a high pressure and an intermediate-low pressure turbine, said gas being circulated through a high temperature nuclear reactor and said steam source by a circulating means rotated by an auxiliary steam turbine means connected to pass a portion of the steam flow to the inlet of the reheating section, and said hot reheat header being connected so that steam may flow from the header to condensing means through the first and second intermediate-low pressure turbines and through first and second bypass lines, said control system comprising,

means to generate a first signal representative of a desired steam flow through the first intermediatelow pressure turbine,

means to generate a second signal representative of a desired steam flow through the second intermediate-low pressure turbine,

pressure detecting means to generate a signal representative of a detected value of steam pressure in the hot reheat header,

means to generate a signal representative of a desired value of steam pressure in the hot reheat header, first valve means connected to govern the steam flow through the first bypass line,

second valve means connected to govern the steam flow through the second bypass line, and

control means responsive to the first and second desired steam tlow signals to position the first and second valve means to govern the steam flows through the first and second bypass lines to maintain a steam flow through the reheating section that is equal to a predetermined minimum flow at times when the total desired steam flow through the first and second intermediate-low pressure turbine is less than such minimum and the detected and desired steam pressure values are equal to a low load pressure value, and responsive to the detected and desired pressure signals when said pressure signals differ to vary the steam flows through the first and second bypass lines to reduce said difference.

18. A control system according to claim 17 wherein the control means includes,

means to generate a first bias signal, and

means to generate a second bias signal, and wherein the first valve means is positioned in accordance with the difference between the first bias signal and the first desired steam flow signal and the second valve means is positioned in accordance with the difference between the second bias signal and the second desired steam flow signal, at times when the detected and desired steam pressure values are equal.

19. A control system according to claim 17 wherein the control means includes,

means to generate a first feedback signal in accordance with the difference between the detected and desired pressure signals, and

means to generate a second feedback signal in accordance with the difference between the detected and desired pressure signals, and wherein the steam flow through the first bypass line is varied in accordance with the first feedback signal, and the steam flow through the second bypass line is varied in accordance with the second feedback signal.

20. A control system according to claim 19 wherein the first feedback signal has a first predetermined proportionality with the difference between the detected and desired pressure signals and the second feedback signal has a second predetermined proportionality with said difference.

21. A control system according to claim 20 wherein said first and second predetermined proportionalities are equal.

22. A control system according to claim 18 wherein the first valve means position is linearly related to the difference between the first bias signal and the first desired steam flow signal, and the second valve means position is linearly related to the difference between the second bias signal and the second desired steam flow signal at times when the detected and desired pressure values are equal, each of said first and second valve means having a linear relationship between steam flow through the valve means and position of the valve means at constant inlet pressure to the valve means.

23. A control system according to claim 22 wherein each of said first and second valve means is closed at times when its respective difference signal is zero.

24. A control system according to claim 17, wherein each of said first and second valve means is positioned to cause a steam flow through its respective bypass line equal to one-half the predetermined minimum steam flow at times when the respective desired steam flow signal is zero and the desired and detected steam pressue values are equal to a predetermined low load pressure value.

25. A system for controlling steam pressure in a hot reheat header connected to receive reheated steam from a reheating section of a steam source that is adapted to derive heat from a flow of reactor coolant gas and to supply superheated and reheated steam to first and second turbine-generators, each of said first and second turbine-generators including at least a high pressure and an intermediate-low pressure turbine, said gas being circulated through a high temperature nuclear reactor and said steam source by a circulating means rotated by an auxiliary steam turbine means connected to pass a portion of the steam flow to the inlet of the reheating section, said hot reheat header being connected so that steam may flow from the hot reheat header to condensing means through the first and second intermediate-low pressure turbines and through first and second condenser bypass lines, with first and second alternate bypass lines connected to pass steam from the hot reheat header to alternate steam discharge means, each of said bypass lines having a valve means therein connected to govern the steam flow throgh the respective line, said control system comprising,

means to generate a first signal representative of a desired steam flow through the first intermediatelow pressure turbine,

means to generate a second signal representative of a desired steam flow through the second intermediate-low pressure turbine,

pressure detecting means to generate a signal representative of a detected value of steam pressure in the hot reheat header,

means to generate a signal representative of a desired value of steam pressure in the hot reheat header, and

control means responsive to the first and second desired steam flow signals to position the first and second condenser bypass line valve means and the first and second alternate bypass line valve means to govern the starn flow through the bypass lines to maintain a steam flow through the reheating section that is equal to a predetermined minimum flow at times when the detected and desired pressure values are equal to a predetermined low load pressure value and the total desired steam flow through the first and second intermediate-low pressure turbines is less than the predetermined minimum flow, and responsive to the detected and desired pressure signals when said signals differ to vary the total bypass steam flow to reduce the difference.

26. A control system according to claim 25 wherein the control means include,

means to generate a first limit signal representative of a limit value of steam flow through the first condenser bypass line, and

means to generate a second limit signal representative of a limit value of steam flow through the second condenser bypass line,

and wherein the first alternate bypass valve means is closed at times when the steam flow through the first condenser bypass line is less than the value represented by the first limit signal, and the second alternate bypass valve means is closed at times when the steam flow through the second condenser bypass line is less than the valve represented by the second limit signal.

27. A control system according to claim 26 wherein the first and second limit signals are generated by first and second function generators, each of said function generators being responsive to the detected pressure signal to generate a limit signal of value inversely proportional to the value of the detected pressure signal.

28. A control system according to claim 26 wherein control means furtherinclude,

means to generate a first bias signal,

means to generate a first feedback signal in accordance with the difference between the detected and desired pressure signals,

means to generate a second bias signal, and

means to generate a second feedback signal in accordance with the difference between the detected and desired pressure signals, and wherein the first condenser bypass valve means is positioned in accordance with a first flow demand signal representative of the sum of the first bias signal with the first feedback signal diminished by the first desired steam flow signal at times when the first flow demand signal is less than the first limit signal, and wherein the second condenser bypass valve means is positioned in accordance with a second flow demand signal representative of the sum of the second bias signal with the second feedback signal diminished by the second desired steam flow signal at times when the second flow demand signal is less than the second flow limit signal.

29. A control system according to claim 28 wherein each of said first and second feedback signals has a predetermined proportionality with the difference between the detected and desired pressure signals.

30. A control system according to claim 28 wherein the first condenser bypass valve means is positioned in accordance with the first limit signal and the first alternate bypass valve means is positioned in accordance with the difference between the first flow demand signal and the first limit signal at times when the first flow demand signal exceeds the first flow limit signal, and wherein the second condenser bypass valve means is positioned in accordance with the second limit signal and the second alternate bypass valve means is positioned in accordance with the difference between the second flow demand signal and the second limit signal at times when the second flow demand signal exceeds the second limit signal.

31. A control system according to claim 30 wherein each of said first and second feedback signals has a predetermined proportionality with the difference between the detected and desired pressure signals 32. A power plant wherein a steam source derives heat from a reactor coolant gas that is circulated through the steam source and a high temperature nuclear reactor, said power plant comprising,

electric generating means,

a steam turbine, said turbine at least including a high pressure turbine element and a lower pressure turbine element, each turbine element having a steam inlet and a steam exhaust, said turbine being rotatably connected to the electric generating means,

a steam source connected to supply superheated steam to the steam inlet of the high pressure turbine element, said steam source including means to reheat a flow of steam through reheating section connected between a steam inlet and the steam outlet,

means to conduct steam from the high pressure turbine steam exhaust to the steam inlet of the reheating section, said means including an auxiliary steam turbine means connected to pass at least a portion of the steam flow to the inlet of the reheating section, said auxiliary steam turbine means being rotatably connected to a means for circulating the reactor coolant gas,

a hot reheat header connected to the steam outlet of the reheating section to receive reheated steam and connected to supply reheated steam to the steam inlet of the lower pressure turbine element,

a condensing means connected to the steam exhaust of the lower pressure turbine element to condense steam discharged therefrom,

a bypass line connected to conduct steam from the hot reheat header to the condensing means,

valve means connected to govern the flow of steam through the bypass line,

means to generate a first signal representative of a desired steam flow through the lower pressure turbine element,

means to generate a second signal representative of a detected pressure value of steam in the hot reheat header,

means to generate a third signal representative of a desired pressure value of steam in the hot reheat header, and

control means responsive to the first signal to position the valve means to govern the steam flow through the bypass line to maintain a steam flow through the reheating section that is equal to a predetermined minimum flow at times when the hot reheat header steam pressure is at a predetermined low load pressure value and the desired steam flow through the lower pressure turbine element is less than the predetermined minimum flow, and responsive to the second and third signals at times when the second and third signals differ to vary the steam flow through the bypass line to reduce the difference.

33. A power plant according to claim 32 wherein the control means include,

means to generate a fourth signal having a predetermined proportionality with the first signal,

means to generate a bias signal,

means responsive to the second and third signals to generate a fifth signal in accordance with the difference between the second and third signals,

means to generate a sixth signal representative of the sum of the fifth signal with the bias signal diminished by the fourth signal, and

means to position the valve means in accordance with the sixth signal.

34. A power plant according to claim 33 wherein the fifth signal has a predetermined proportionality with the difference between the second and third signals.

35. A power plant according to claim 33 wherein the valve means is characterized by a linear relationship between steam flow through the valve means and position of the valve means at constant inlet pressure to the valve means, and the positioning means positions the valve means at a position which is linearly related to the sixth signal.

36. A power plant wherein a steam source derives heat from a reactor coolant gas that is circulated through the steam source and a high temperature nuclear reactor, said power plant comprising,

electric generating means,

a steam turbine, said turbine at least including a high pressure turbine element and a lower pressure turbine element, each turbine element having a steam inlet and a steam exhaust, said turbine being rotatably connected to the electric generating means,

a steam source connected to supply superheated steam to the steam inlet of the high pressure turbine element, said steam source including means to reheat a flow of steam through a reheating section connected between a steam inlet and a steam oulet,

means to conduct steam from the high pressure turbine element exhaust to the steam inlet of the reheating section, said means including an auxiliary steam turbine means connected to pass at least a portion of the steam flow to the inlet of the reheating section, said auxiliary steam turbine means being rotatably connected to a means for circulating the reactor coolant gas,

a hot reheat header connected to the steam outlet of the reheating section and connected to supply reheated steam to the steam inlet of the lower pressure turbine element,

a condensing means connected to the steam exhaust of the lower pressure turbine element to condense steam discharged therefrom,

a condenser bypass line connected to conduct steam from the hot reheat header to the condensing means,

first valve means connected to govern the flow of steam through the condenser bypass line,

an alternate bypass line connected to conduct steam from the hot reheat header to an alternate steam discharge means,

second valve means connected to govern the flow of steam through the alternate bypass line,

means to generate a first signal representative of a desired steam flow through the lower pressure turbine element,

means to generate a second signal representative of a detected pressure value of steam in the hot reheat header,

means to generate a third signal representative of a desired pressure value of steam in the hot reheat header, and

control means responsive to the first signal to position the first and second valve means to govern the steam flows through the bypass lines to maintain a steam flow through the reheating section that is equal to a predetermined minimum fiow at times when the hot reheat header is at a predetermined low load pressure value and the desired steam flow through the lower pressure turbine element is less than the predetermined minimum flow, and responsive to the second and third signals at times when the second and third signals differ to vary the steam flow through one of the bypass lines to reduce the difference.

37. A power plant according to claim 36 wherein the steam flow through one of the bypass lines is varied in accordance with the difference between the detected and desired steam pressure values.

38. A power plant, according to claim 36 wherein the control means include means to generate a signal representative of a limit value of steam flow through the condenser bypass line, and the second valve means is closed at times when the steam flow through the condenser bypass line is less than the limit value.

39. A power plant according to claim 38 wherein the control means further include,

means to generate a bias signal,

means to generate a feedback signal in accordance with the difference between the second and third signals, and

means to generate a flow demand signal representative of the sum of the bias signal with the feedback signal diminished by the first signal, and wherein the first valve means is positioned in accordance with the flow demand signal at times when the flow demand signal is less than the limit signal.

40. A power plant according to claim 39 wherein the first valve means is positioned in accordance with the limit signal and the second valve means is positioned in accordance with the difference between the flow demand signal and the limit signal at times when the flow demand signal exceeds the limit signal.

41. A power plant according to claim 40 wherein the feedback signal has a predetermined proportionality with the difference between the second and third signals.

42. A power plant wherein a steam source derives heat from a reactor coolant gas that is circulated through the steam source and a high temperature nuclear reactor, said steam source being connected to supply superheated and reheated steam to first and second turbine-generators, said power plant comprising,

electric generating means,

first and second steam turbines, each of said first and second steam turbines including at least a high pressure turbine element and a lower pressure turbine element, each turbine element having a steam inlet and a steam exhaust, said steam turbines being rotatably connected to the electric generating means,

steam source connected to supply superheated steam to the steam inlets of the high pressure turbine elements, said steam source including means to reheat a flow of steam through a reheating section connected between a steam inlet and a steam outlet, means to conduct steam from the high pressure turbine element steam exhausts to the steam inlet of the reheating section, said means including an auxiliary steam turbine means connected to pass at least a portion of the steam flow to the inlet of the reheating section, said auxiliary steam turbine means being rotatably connected to a means for circulating the reactor coolant gas,

a hot reheat header connected to the steam outlet of the reheating section and connected to supply reheated steam to the steam inlets of the lower pres sure turbine elements,

condensing means connected to the steam exhausts of the lower pressure turbine elements to condense steam discharged therefrom,

first and second bypass lines connected to conduct steam from the hot reheat header to the condensmg means,

first and second valve means connected to govern the steam flows through the respective first and second bypass lines,

means to generate a first signal representative of a desired steam flow through the first lower pressure turbine element,

means to generate a second signal representative of a desired steam flow through the second lower pressure turbine element,

means to generate a third signal representative of a detected pressure value of steam in the hot reheat header,

means to generate a fourth signal representative of a desired pressure value of steam in the hot reheat header, and

control means responsive to the first and second signals to position the first and second valve means to govern the steam flows through the first and second bypass lines to maintain a steam flow through the reheating section that is equal to a predetermined minimum flow at times when the hot reheat header steam pressure is at a predetermined low load pressure value and the total desired steam flow through the first and second lower pressure turbine elements is less than the predetermined minimum flow, and responsive to the third and fourth signals when the third and fourth signals differ to vary the total steam flow through the first and second bypass lines to reduce the difference.

43. A power plant according to claim 42 wherein the total steam flow through the first and second bypass lines is varied in accordance with the difference between the third and fourth signals.

44. A power plant according to claim 43 wherein the total steam flow through the first and second bypass lines is varied in predetermined proportionality with the difference between the third and fourth signals.

45. A power plant according to claim 42 wherein the control means include,

means to generate a first bias signal,

means to generate a second bias signal,

means to generate a first feedback signal having a predetermined proportionality with the difference between the third and fourth signals,

means to generate a second feedback signal having a predetermined proportionality with the difference between the third and fourth signals,

means to position the first valve means in accordance with the sum of the first bias signal with the first feedback signal diminished by the first desired steam flow signal, and

means to position the second valve means in accordance with the sum of the second bias signal with the second feedback signal diminished by the second desired steam flow signal.

46. A power plant according to claim 45 wherein each of said first and second valve means is characterized by a linear relationship between steam flow through the valve means and position of the valve means at constant inlet pressure to the valve means, and wherein the position of each of said valve means is linearly related to the sum of the respective bias signal with the respective feedback signal, diminished by the respective desired steam flow signal.

47. A method of operating a dual turbine-generator power plant which includes a steam source adapted to derive heat from the coolant gas of a high temperature nuclear reactor, said steam source being arranged to supply steam to first and second turbine-generators,

each of said first and second turbine generators including a high pressure turbine connected to utilize superheated steam and a lower pressure turbine connected to utilize reheated steam from a hot reheat header connected to the outlet of a reheating section of the steam source, said coolant gas being circulated through the reactor and the steam source by a circulating means driven by an auxiliary steam turbine means connected to pass a portion of the steam flow to the inlet of the reheating section, with first and second bypass lines connected to pass steam from the hot reheat header to condensing means connected to the exhausts of the first and second lower pressure turbines, each of said first and second bypass lines having a bypass valve means therein connected to govern the flow of steam therethrough, said method comprising,

measuring the pressure value of steam in the hot reheat header,

generating a first signal representative of the measured pressure value,

generating a second signal representative of the pressure value of steam in the hot reheat header prevailing at a point of time immediately preceding a turbine trip,

tripping one of said first and second turbines,

holding closed the bypass valve means in the bypass line associated with the operating turbinegenerator, and

varying the position of the bypass valve means in the bypass line associated with the tripped turbine to reduce a difference between the first and second signals.

48. A method according to claim 47 wherein the position of the bypass valve means in the bypass line associated with the tripped turbine is varied in accordance with the difference between the first and second signals,

49. A method according to claim 48 wherein the bypass valve means position is varied in predetermined proportion to the difference between the first and second signals. 

1. A system for controlling steam pressure in a hot reheat header connected to receive reheated steam from a reheating section of a steam source that is adapted to derive heat from a flow of reactor coolant gas and to supply superheated and reheated steam to a turbine-generator including at least a high pressure and an intermediate-low pressure turbine, said gas being circulated through a high temperature nuclear reactor and said steam source by a circulating means rotated by an auxiliary steam turbine means connected to pass a portion of the steam which flows to the inlet of the reheating section, and said hot reheat header being connected so that steam may flow from the header to a condensing means through a first path comprising the intermediate-low pressure turbine and through a second path comprising a condenser bypass line, said control system comprising, valve means connected to govern the steam flow through the condenser bypass line, means to generate a first signal representative of a desired steam flow through the intermediate-low pressure turbine, pressure detecting means to generate a second signal representative of a detected value of steam pressure in the hot reheat header, means to generate a third signal representative of a desired value of steam pressure in the hot reheat header, and control means responsive to the first signal to position the valve means to govern the steam flow through the condenser bypass line to maintain a steam flow through the reheating section that is equal to a predetermined minimum flow at times when the detected and desired pressure values are equal to a predetermined low load pressure value and the desired steam flow through the intermediate-low pressure turbine is less than the predetermined minimum flow, and responsive to the second and third signals when the second and third signals are different to vary the steam flow through the condenser bypass line in proportion to the difference of the second and third signals to reduce said difference.
 2. A control system according to claim 1 wherein the control means include, means to generate a fourth signal having a predetermined proportionality with the difference between the second and third signals, means to generate a fifth signal having a predetermined proportionality with the first signal, means to generate a sixth signal representative of the fourth signal diminished by the fifth signal, and means to position the valve means in accordance with the sixth signal.
 3. A control system according to claim 2 wherein the means to generate the fourth signal comprise, a comparator to generate an output signal representative of the difference between the second signal and the third signal, and a proportional controller responsive to the output signal of the comparator to generate the fourth signal, said fourth signal having a predetermined proportionality with the comparator output signal.
 4. A control system according to claim 1 wherein the valve means is characterized by a linear relationship between steam flow through the valve means and position of the valve means at constant inlet pressure to the valve means, and wherein the associated positioning means positions the valve means at a position which is linearly related to the sixth signal.
 5. A control system according to claim 2 wherein the control means further include means to generate a bias signal, and wherein the sixth signal represents the sum of the bias signal with the fourth signal, diminished by the fifth signal.
 6. A control system according to claim 5 wherein the valve means is characterized by a linear relationship between steam flow through the valve means and position of the valve means at constant inlet pressure to the valve means, and wherein the associated positioning means positions the valve means at a position which is linearly related to the sixth signal.
 7. A system for controlling steam pressure in a hot reheat header connected to receive steam from a reheating section of a steam source that is adapted to derive heat from a flow of reactor coolant gas and to supply superheated and reheated steam to a turbine-generator including at least a high pressure and an intermediate-low pressure turbine, said gas being circulated through a high temperature nuclear reactor and said steam source by a circulating means rotated by an auxiliary steam turbine means connected to pass a portion of the steam which flows to the inlet of the reheating section, said hot reheat header being connected so that steam may flow from the header to a condensing means through a first path comprising the intermediate-low pressure turbine and through a second path comprising a condenser bypass line, an alternate bypass line being connected to pass steam from the hot reheat header to an alternate steam discharge means, said control system comprising, first valve means connected to govern the steam flow through the condenser bypass line, second valve means connected to govern the steam flow through the alternate bypass line, means to generate a first signal representative of a desired steam flow through the intermediate-low pressure turbine, pressure detecting means to generate a second signal representative of a detected value of steam pressure in the hot reheat header, means to generate a third signal representative of a desired value of steam pressure in the hot reheat header, and control means responsive to the first signal to position the first and second valve means to govern the steam flows through said bypass lines to maintain a steam flow through the reheating section that is equal to a predetermined minimum flow at times when the detected and desired pressure valves are equal to a predetermined low load pressure value and the desired steam flow through the intermediate-low pressure turbine is less than the predetermined minimum flow, and responsive to the second and third signals at times when the second and third signals are different to vary the total steam flow through said bypass lines to reduce said difference.
 8. A control system according to claim 7 wherein the control means includes means to generate a signal representative of a limit value of steam flow through the condenser bypass line, and wherein the second valve means is positioned to regulate the condenser bypass line flow at the limit value, the steam flow through the alternate bypass line being varied to reduce said pressure difference at times when the condenser bypass line flow is so regulated.
 9. A control system according to claim 8, wherein the flow limit signal is generated in accordance with the second signal, and the value of the limit signal varies inversely with the detected value of steam pressure in the hot reheat header.
 10. A control system according to claim 9 wherein the means for generating the limit signal is a function generator, said function generator having the second signal as an input signal and generating the limit signal as an output signal.
 11. A control system according to claim 7 wherein the control means include, means to generate a fourth signal in accordance with the difference between the second and third signals, means to generate a fifth signal having a predetermined proportionality with the first signal, means to generate a bias signal, means to generate a sixth signal representative of the sum of the bias signal with the fourth signal diminished by the fifth signal, means to generate a signal representative of a limit valve of steam flow through the condenser bypass line, and means to position the first and second valve means in response to the sixth signal and the limit signal.
 12. A control system according to claim 11 wherein the fourth signal has a preDetermined proportionality with the difference between the second and third signals.
 13. A control system according to claim 11 wherein the means to position the first and second valve means include, selection means responsive to the sixth signal and to the flow limit signal to select the lower of said signals and transmit the selected signal to a means for positioning the first valve means and to a comparator, means to position the first valve means in accordance with the selected signal, a comparator to generate an output signal representative of the difference between the sixth signal and the selected signal, and means to position the second valve means in accordance with the output signal of the comparator.
 14. A control system according to claim 13 wherein the second valve means is closed at times when the sixth signal is lower than the limit signal.
 15. A control system according to claim 14 wherein the first valve means is closed when the sixth signal is zero.
 16. A control system according to claim 13 wherein each of said first and second valve means is characterized by a linear relationship between steam flow through the valve means and position of the valve means at constant differential pressure across the valve means, and wherein the associated positioning means positions the valve means at a position which is linearly related to the signal to which the positioning means is responsive.
 17. A system for controlling steam pressure in a hot reheat header connected to receive reheated steam from a reheating section of a steam source which is adapted to derive heat from a flow of reactor coolant gas and to supply superheated and reheated steam to first and second turbine-generators, each of said first and second turbine-generators including at least a high pressure and an intermediate-low pressure turbine, said gas being circulated through a high temperature nuclear reactor and said steam source by a circulating means rotated by an auxiliary steam turbine means connected to pass a portion of the steam flow to the inlet of the reheating section, and said hot reheat header being connected so that steam may flow from the header to condensing means through the first and second intermediate-low pressure turbines and through first and second bypass lines, said control system comprising, means to generate a first signal representative of a desired steam flow through the first intermediate-low pressure turbine, means to generate a second signal representative of a desired steam flow through the second intermediate-low pressure turbine, pressure detecting means to generate a signal representative of a detected value of steam pressure in the hot reheat header, means to generate a signal representative of a desired value of steam pressure in the hot reheat header, first valve means connected to govern the steam flow through the first bypass line, second valve means connected to govern the steam flow through the second bypass line, and control means responsive to the first and second desired steam flow signals to position the first and second valve means to govern the steam flows through the first and second bypass lines to maintain a steam flow through the reheating section that is equal to a predetermined minimum flow at times when the total desired steam flow through the first and second intermediate-low pressure turbine is less than such minimum and the detected and desired steam pressure values are equal to a low load pressure value, and responsive to the detected and desired pressure signals when said pressure signals differ to vary the steam flows through the first and second bypass lines to reduce said difference.
 18. A control system according to claim 17 wherein the control means includes, means to generate a first bias signal, and means to generate a second bias signal, and wherein the first valve means is positioned in accordance with the difference between the first bias signal and the first desired steam flow signal and the second valve means is positioned in accordance with the difference between the second bias signal and the second desired steam flow signal, at times when the detected and desired steam pressure values are equal.
 19. A control system according to claim 17 wherein the control means includes, means to generate a first feedback signal in accordance with the difference between the detected and desired pressure signals, and means to generate a second feedback signal in accordance with the difference between the detected and desired pressure signals, and wherein the steam flow through the first bypass line is varied in accordance with the first feedback signal, and the steam flow through the second bypass line is varied in accordance with the second feedback signal.
 20. A control system according to claim 19 wherein the first feedback signal has a first predetermined proportionality with the difference between the detected and desired pressure signals and the second feedback signal has a second predetermined proportionality with said difference.
 21. A control system according to claim 20 wherein said first and second predetermined proportionalities are equal.
 22. A control system according to claim 18 wherein the first valve means position is linearly related to the difference between the first bias signal and the first desired steam flow signal, and the second valve means position is linearly related to the difference between the second bias signal and the second desired steam flow signal at times when the detected and desired pressure values are equal, each of said first and second valve means having a linear relationship between steam flow through the valve means and position of the valve means at constant inlet pressure to the valve means.
 23. A control system according to claim 22 wherein each of said first and second valve means is closed at times when its respective difference signal is zero.
 24. A control system according to claim 17, wherein each of said first and second valve means is positioned to cause a steam flow through its respective bypass line equal to one-half the predetermined minimum steam flow at times when the respective desired steam flow signal is zero and the desired and detected steam pressue values are equal to a predetermined low load pressure value.
 25. A system for controlling steam pressure in a hot reheat header connected to receive reheated steam from a reheating section of a steam source that is adapted to derive heat from a flow of reactor coolant gas and to supply superheated and reheated steam to first and second turbine-generators, each of said first and second turbine-generators including at least a high pressure and an intermediate-low pressure turbine, said gas being circulated through a high temperature nuclear reactor and said steam source by a circulating means rotated by an auxiliary steam turbine means connected to pass a portion of the steam flow to the inlet of the reheating section, said hot reheat header being connected so that steam may flow from the hot reheat header to condensing means through the first and second intermediate-low pressure turbines and through first and second condenser bypass lines, with first and second alternate bypass lines connected to pass steam from the hot reheat header to alternate steam discharge means, each of said bypass lines having a valve means therein connected to govern the steam flow throgh the respective line, said control system comprising, means to generate a first signal representative of a desired steam flow through the first intermediate-low pressure turbine, means to generate a second signal representative of a desired steam flow through the second intermediate-low pressure turbine, pressure detecting means to generate a signal representative of a detected value of steam pressure in the hot reheat header, means to generate a signal representative of a desired value of steam pressure in the hot rehEat header, and control means responsive to the first and second desired steam flow signals to position the first and second condenser bypass line valve means and the first and second alternate bypass line valve means to govern the stam flow through the bypass lines to maintain a steam flow through the reheating section that is equal to a predetermined minimum flow at times when the detected and desired pressure values are equal to a predetermined low load pressure value and the total desired steam flow through the first and second intermediate-low pressure turbines is less than the predetermined minimum flow, and responsive to the detected and desired pressure signals when said signals differ to vary the total bypass steam flow to reduce the difference.
 26. A control system according to claim 25 wherein the control means include, means to generate a first limit signal representative of a limit value of steam flow through the first condenser bypass line, and means to generate a second limit signal representative of a limit value of steam flow through the second condenser bypass line, and wherein the first alternate bypass valve means is closed at times when the steam flow through the first condenser bypass line is less than the value represented by the first limit signal, and the second alternate bypass valve means is closed at times when the steam flow through the second condenser bypass line is less than the valve represented by the second limit signal.
 27. A control system according to claim 26 wherein the first and second limit signals are generated by first and second function generators, each of said function generators being responsive to the detected pressure signal to generate a limit signal of value inversely proportional to the value of the detected pressure signal.
 28. A control system according to claim 26 wherein control means further include, means to generate a first bias signal, means to generate a first feedback signal in accordance with the difference between the detected and desired pressure signals, means to generate a second bias signal, and means to generate a second feedback signal in accordance with the difference between the detected and desired pressure signals, and wherein the first condenser bypass valve means is positioned in accordance with a first flow demand signal representative of the sum of the first bias signal with the first feedback signal diminished by the first desired steam flow signal at times when the first flow demand signal is less than the first limit signal, and wherein the second condenser bypass valve means is positioned in accordance with a second flow demand signal representative of the sum of the second bias signal with the second feedback signal diminished by the second desired steam flow signal at times when the second flow demand signal is less than the second flow limit signal.
 29. A control system according to claim 28 wherein each of said first and second feedback signals has a predetermined proportionality with the difference between the detected and desired pressure signals.
 30. A control system according to claim 28 wherein the first condenser bypass valve means is positioned in accordance with the first limit signal and the first alternate bypass valve means is positioned in accordance with the difference between the first flow demand signal and the first limit signal at times when the first flow demand signal exceeds the first flow limit signal, and wherein the second condenser bypass valve means is positioned in accordance with the second limit signal and the second alternate bypass valve means is positioned in accordance with the difference between the second flow demand signal and the second limit signal at times when the second flow demand signal exceeds the second limit signal.
 31. A control system according to claim 30 wherein each of said first and second feedback signals has a predetermined proportionality with the difference between the detected and desIred pressure signals
 32. A power plant wherein a steam source derives heat from a reactor coolant gas that is circulated through the steam source and a high temperature nuclear reactor, said power plant comprising, electric generating means, a steam turbine, said turbine at least including a high pressure turbine element and a lower pressure turbine element, each turbine element having a steam inlet and a steam exhaust, said turbine being rotatably connected to the electric generating means, a steam source connected to supply superheated steam to the steam inlet of the high pressure turbine element, said steam source including means to reheat a flow of steam through reheating section connected between a steam inlet and the steam outlet, means to conduct steam from the high pressure turbine steam exhaust to the steam inlet of the reheating section, said means including an auxiliary steam turbine means connected to pass at least a portion of the steam flow to the inlet of the reheating section, said auxiliary steam turbine means being rotatably connected to a means for circulating the reactor coolant gas, a hot reheat header connected to the steam outlet of the reheating section to receive reheated steam and connected to supply reheated steam to the steam inlet of the lower pressure turbine element, a condensing means connected to the steam exhaust of the lower pressure turbine element to condense steam discharged therefrom, a bypass line connected to conduct steam from the hot reheat header to the condensing means, valve means connected to govern the flow of steam through the bypass line, means to generate a first signal representative of a desired steam flow through the lower pressure turbine element, means to generate a second signal representative of a detected pressure value of steam in the hot reheat header, means to generate a third signal representative of a desired pressure value of steam in the hot reheat header, and control means responsive to the first signal to position the valve means to govern the steam flow through the bypass line to maintain a steam flow through the reheating section that is equal to a predetermined minimum flow at times when the hot reheat header steam pressure is at a predetermined low load pressure value and the desired steam flow through the lower pressure turbine element is less than the predetermined minimum flow, and responsive to the second and third signals at times when the second and third signals differ to vary the steam flow through the bypass line to reduce the difference.
 33. A power plant according to claim 32 wherein the control means include, means to generate a fourth signal having a predetermined proportionality with the first signal, means to generate a bias signal, means responsive to the second and third signals to generate a fifth signal in accordance with the difference between the second and third signals, means to generate a sixth signal representative of the sum of the fifth signal with the bias signal diminished by the fourth signal, and means to position the valve means in accordance with the sixth signal.
 34. A power plant according to claim 33 wherein the fifth signal has a predetermined proportionality with the difference between the second and third signals.
 35. A power plant according to claim 33 wherein the valve means is characterized by a linear relationship between steam flow through the valve means and position of the valve means at constant inlet pressure to the valve means, and the positioning means positions the valve means at a position which is linearly related to the sixth signal.
 36. A power plant wherein a steam source derives heat from a reactor coolant gas that is circulated through the steam source and a high temperature nuclear reactor, said power plant comprising, electric generating means, a steam turbine, said turbine at least including a high pressure turbine element and a lower Pressure turbine element, each turbine element having a steam inlet and a steam exhaust, said turbine being rotatably connected to the electric generating means, a steam source connected to supply superheated steam to the steam inlet of the high pressure turbine element, said steam source including means to reheat a flow of steam through a reheating section connected between a steam inlet and a steam oulet, means to conduct steam from the high pressure turbine element exhaust to the steam inlet of the reheating section, said means including an auxiliary steam turbine means connected to pass at least a portion of the steam flow to the inlet of the reheating section, said auxiliary steam turbine means being rotatably connected to a means for circulating the reactor coolant gas, a hot reheat header connected to the steam outlet of the reheating section and connected to supply reheated steam to the steam inlet of the lower pressure turbine element, a condensing means connected to the steam exhaust of the lower pressure turbine element to condense steam discharged therefrom, a condenser bypass line connected to conduct steam from the hot reheat header to the condensing means, first valve means connected to govern the flow of steam through the condenser bypass line, an alternate bypass line connected to conduct steam from the hot reheat header to an alternate steam discharge means, second valve means connected to govern the flow of steam through the alternate bypass line, means to generate a first signal representative of a desired steam flow through the lower pressure turbine element, means to generate a second signal representative of a detected pressure value of steam in the hot reheat header, means to generate a third signal representative of a desired pressure value of steam in the hot reheat header, and control means responsive to the first signal to position the first and second valve means to govern the steam flows through the bypass lines to maintain a steam flow through the reheating section that is equal to a predetermined minimum flow at times when the hot reheat header is at a predetermined low load pressure value and the desired steam flow through the lower pressure turbine element is less than the predetermined minimum flow, and responsive to the second and third signals at times when the second and third signals differ to vary the steam flow through one of the bypass lines to reduce the difference.
 37. A power plant according to claim 36 wherein the steam flow through one of the bypass lines is varied in accordance with the difference between the detected and desired steam pressure values.
 38. A power plant, according to claim 36 wherein the control means include means to generate a signal representative of a limit value of steam flow through the condenser bypass line, and the second valve means is closed at times when the steam flow through the condenser bypass line is less than the limit value.
 39. A power plant according to claim 38 wherein the control means further include, means to generate a bias signal, means to generate a feedback signal in accordance with the difference between the second and third signals, and means to generate a flow demand signal representative of the sum of the bias signal with the feedback signal diminished by the first signal, and wherein the first valve means is positioned in accordance with the flow demand signal at times when the flow demand signal is less than the limit signal.
 40. A power plant according to claim 39 wherein the first valve means is positioned in accordance with the limit signal and the second valve means is positioned in accordance with the difference between the flow demand signal and the limit signal at times when the flow demand signal exceeds the limit signal.
 41. A power plant according to claim 40 wherein the feedback signal has a predetermined proportionality with the difference between the second and third signals.
 42. A power plant wherein a steam source derives heat from a reactor coolant gas that is circulated through the steam source and a high temperature nuclear reactor, said steam source being connected to supply superheated and reheated steam to first and second turbine-generators, said power plant comprising, electric generating means, first and second steam turbines, each of said first and second steam turbines including at least a high pressure turbine element and a lower pressure turbine element, each turbine element having a steam inlet and a steam exhaust, said steam turbines being rotatably connected to the electric generating means, a steam source connected to supply superheated steam to the steam inlets of the high pressure turbine elements, said steam source including means to reheat a flow of steam through a reheating section connected between a steam inlet and a steam outlet, means to conduct steam from the high pressure turbine element steam exhausts to the steam inlet of the reheating section, said means including an auxiliary steam turbine means connected to pass at least a portion of the steam flow to the inlet of the reheating section, said auxiliary steam turbine means being rotatably connected to a means for circulating the reactor coolant gas, a hot reheat header connected to the steam outlet of the reheating section and connected to supply reheated steam to the steam inlets of the lower pressure turbine elements, condensing means connected to the steam exhausts of the lower pressure turbine elements to condense steam discharged therefrom, first and second bypass lines connected to conduct steam from the hot reheat header to the condensing means, first and second valve means connected to govern the steam flows through the respective first and second bypass lines, means to generate a first signal representative of a desired steam flow through the first lower pressure turbine element, means to generate a second signal representative of a desired steam flow through the second lower pressure turbine element, means to generate a third signal representative of a detected pressure value of steam in the hot reheat header, means to generate a fourth signal representative of a desired pressure value of steam in the hot reheat header, and control means responsive to the first and second signals to position the first and second valve means to govern the steam flows through the first and second bypass lines to maintain a steam flow through the reheating section that is equal to a predetermined minimum flow at times when the hot reheat header steam pressure is at a predetermined low load pressure value and the total desired steam flow through the first and second lower pressure turbine elements is less than the predetermined minimum flow, and responsive to the third and fourth signals when the third and fourth signals differ to vary the total steam flow through the first and second bypass lines to reduce the difference.
 43. A power plant according to claim 42 wherein the total steam flow through the first and second bypass lines is varied in accordance with the difference between the third and fourth signals.
 44. A power plant according to claim 43 wherein the total steam flow through the first and second bypass lines is varied in predetermined proportionality with the difference between the third and fourth signals.
 45. A power plant according to claim 42 wherein the control means include, means to generate a first bias signal, means to generate a second bias signal, means to generate a first feedback signal having a predetermined proportionality with the difference between the third and fourth signals, means to generate a second feedback signal having a predetermined proportionality with the difference between the third and fourth signals, means to position the first valve means in accordance with the sum of the first bias signal with the first feedback signal diminisheD by the first desired steam flow signal, and means to position the second valve means in accordance with the sum of the second bias signal with the second feedback signal diminished by the second desired steam flow signal.
 46. A power plant according to claim 45 wherein each of said first and second valve means is characterized by a linear relationship between steam flow through the valve means and position of the valve means at constant inlet pressure to the valve means, and wherein the position of each of said valve means is linearly related to the sum of the respective bias signal with the respective feedback signal, diminished by the respective desired steam flow signal.
 47. A method of operating a dual turbine-generator power plant which includes a steam source adapted to derive heat from the coolant gas of a high temperature nuclear reactor, said steam source being arranged to supply steam to first and second turbine-generators, each of said first and second turbine generators including a high pressure turbine connected to utilize superheated steam and a lower pressure turbine connected to utilize reheated steam from a hot reheat header connected to the outlet of a reheating section of the steam source, said coolant gas being circulated through the reactor and the steam source by a circulating means driven by an auxiliary steam turbine means connected to pass a portion of the steam flow to the inlet of the reheating section, with first and second bypass lines connected to pass steam from the hot reheat header to condensing means connected to the exhausts of the first and second lower pressure turbines, each of said first and second bypass lines having a bypass valve means therein connected to govern the flow of steam therethrough, said method comprising, measuring the pressure value of steam in the hot reheat header, generating a first signal representative of the measured pressure value, generating a second signal representative of the pressure value of steam in the hot reheat header prevailing at a point of time immediately preceding a turbine trip, tripping one of said first and second turbines, holding closed the bypass valve means in the bypass line associated with the operating turbine-generator, and varying the position of the bypass valve means in the bypass line associated with the tripped turbine to reduce a difference between the first and second signals.
 48. A method according to claim 47 wherein the position of the bypass valve means in the bypass line associated with the tripped turbine is varied in accordance with the difference between the first and second signals.
 49. A method according to claim 48 wherein the bypass valve means position is varied in predetermined proportion to the difference between the first and second signals. 